Causes

Atrioventricular canal defect occurs before birth when a baby's heart is developing. Doctors aren't exactly sure what causes atrioventricular canal defect. Down syndrome might increase a person's risk of this congenital heart defect.

The normal heart

The heart is divided into four chambers, two on the right and two on the left.

The right side of the heart moves blood into vessels that lead to the lungs, where the blood receives oxygen. The oxygen-rich blood flows back to your heart's left side and into the body's main artery (aorta). From there, the blood flows to the rest of your body.

Valves control the flow of blood into and out of the chambers of your heart. These heart valves open to let blood in and close to keep blood from flowing backward.

What happens in atrioventricular canal defect

Partial atrioventricular canal defect:

• A hole in the heart's wall (septum) separates the upper chambers (atria).
• Often the valve between the upper and lower left chambers (mitral valve) is abnormal and leaks blood (mitral valve regurgitation).

Complete atrioventricular canal defect:

• There's a large hole in the center of the heart where the walls between the upper chambers (atria) and the lower chambers (ventricles) meet. Oxygen-rich and oxygen-poor blood mix through that hole.
• Instead of separate valves on the right and left, there's one large valve between the upper and lower chambers.
• The abnormal valve leaks blood into the ventricles.
• The heart is forced to work harder and becomes larger.